This project focused on model heterogeneous catalysts for the gas-phase hydrogenation and electrocatalytic reduction of carbon dioxide as well as the generation of green hydrogen from renewable-electricity driven water splitting. We have addressed the systematic design of catalytically active model nanoparticle pre-catalysts with narrow size and shape distributions and tunable oxidation state, and in situ and operando structural, chemical, and reactivity characterization of such model catalysts as a function of the reaction environment.
Key components of the present project involved:
1. The chemical synthesis of well-defined pre-catalyst materials with control on the initial size, shape, chemical composition and dispersion on the support could be achieved.
2. The detailed monitoring of the evolution of the structure, surface and bulk composition of the above nanoparticle catalysts under reaction conditions using spectroscopic and microscopic methods. These challenging measurements were done operando, i.e. while the nanoscale catalysts were “at work”. A multi-technique approach was employed in order to unveil the complex nature of nanocatalysts.
3. Correlations could be established between the structure, chemical state and reactivity of the nanoparticles under different applied external stimulus (e.g. electrical potential and/or chemical environment) and as a function of the reaction time.
Our comprehensive research lead to a total of 72 publications in high impact scientific journals. A few results are highlighted below.
In the field of thermal catalysis, we investigated the synthesis of methanol from carbon dioxide and hydrogen. Here, we developed synthetic routes to produce size-selected mono- and bimetallic nanoparticles of various compositions (e.g. copper-zinc, copper-nickel). These nanoparticles served as excellent model systems to study the thermal hydrogenation of carbon dioxide. The combination of surface science methods, operando spectroscopy and reactivity measurements allowed us to look in detail at the structural and chemical state of the active catalysts and their evolution under reaction conditions.
In particular, we looked at particle-support interactions for copper-zinc particles on different support materials. We found that zinc oxide is not needed as part of the support, but that minute amounts of zinc within the core of copper-rich copper-zinc particles can still activate the catalyst for methanol synthesis. Moreover, zinc can migrate into a support material (e.g. aluminium oxide), leading to dimethylether selectivity instead of methanol, with the ratio of both products being tunable as a function of the zinc content inside the copper-zinc nanoparticles. We learnt that the choice of the support material can directly affect the catalytic performance of the system [1].
In the field of electrocatalysis, we comprehensively investigated the electrochemical reduction of carbon dioxide under working conditions. By studying model copper single crystal surfaces, we revealed the inactivity of defect-free copper surfaces that were prepared under ultrahigh vacuum conditions. Defect-free copper surfaces favor the production of hydrogen instead of organic multicarbon products [2]. Only the appearance of mesostructural inhomogeneities like step bunches as well as nanoscale roughness allowed the formation of multicarbon products. Moreover, even for identically oriented single crystal surfaces, we could demonstrate that minute changes in the initial structure of Cu surfaces have a drastic effect in their selectivity [3]. This insight broke the paradigm of early studies that distinct surface terminations of copper would lead to a preferred formation of hydrocarbons and fuels with two or more carbon atoms (C2+ products).
The catalytic role of cationic copper species had not yet been unambiguously identified. However, we showed that the continuous regeneration of distorted amorphous Cu1+/2+ domains boost the ethanol formation in an electrically pulsed reaction [4].
Furthermore, we accomplished to perform electron microscopy under electrocatalytic conditions in the liquid phase, strongly contributing to the development of this method [5].
References
[1.] Kordus et al. J. Am. Chem. Soc. 2023, 145, 3016.
[2.] Scholten et al. Angew. Chem. Int. Ed. 2021, 60, 19169.
[3.] Nguyen et al. ACS Energy Lett. 2024, 644.
[4.] Timoshenko et al. Nat. Catal. 2022, 5, 259.
[5.] Arán-Ais et al. Nat. Commun. 2020, 11, 3489.
